A manufacturing method of a thin film transistor includes the following steps. A substrate is provided first. A semiconductor layer is then formed on the substrate. Next, a photoresist pattern including a middle portion and two peripheral portions is formed on the semiconductor layer. The middle portion is disposed between two peripheral portions, and the thickness of the middle portion is greater than each of the peripheral portions. Next, an etching process is performed on the semiconductor layer for forming a patterned semiconductor layer. A photoresist ashing process is then performed to remove at least the peripheral portions of the photoresist pattern to form a channel defining photoresist pattern and expose two portions of the patterned semiconductor layer. Next, the patterned semiconductor layer is treated to form a semiconductor portion and two conductor portions. The channel defining photoresist pattern is then removed.